The problem that arises is that of estimating and correcting frequency errors in a CDMA communications network. Frequency errors are due to various physical phenomena that involve the quartz crystal of an oscillator of a mobile radio terminal, thereby causing a frequency offset. These phenomena can be associated with temperature, gradual deterioration of the crystal at the atomic level, etc.
Because of these phenomena, the oscillator of the mobile radio terminal is not always synchronized to that of the base station. Frequency errors can then degrade performance in terms of the demodulation that recovers binary information, and can therefore lead to loss of data. Frequency errors also have a direct impact on performance in terms of propagation channel estimation.
Accordingly, with the aim of obtaining good frequency synchronization between the base station and the mobile radio terminal, a very accurate crystal can be used in the local oscillator of the terminal. However, the more accurate the crystal, the more costly. An oscillator provided with this kind of crystal is therefore too costly to envisage its use in the mass production of mobile radio terminals.
Another approach is to use frequency error correction for the base band part of the signal, i.e. the digital part of the signal, retaining a low-cost local oscillator using a conventional crystal for processing the radio-frequency signal. In this particular context, which is that of the invention, frequency error correction techniques exist already and are well known to the person skilled in the art. Such techniques can be divided into two types.
Firstly, there are techniques based on frequency domain analysis which highlights the frequencies of a complex signal and determines the amplitudes and phases of the corresponding partial signals. Thus a power spectrum is calculated by means of a fast Fourier transform. The offset of the spectrum relative to the 0 Hz frequency reference is evaluated from the power spectrum estimate obtained. It is then a question of centering the spectrum, with the aim of compensating the offset caused by the frequency error. Finally, an inverse fast Fourier transform effects a return to the time domain for continued processing of the corresponding signal.
Secondly, there are frequency error correction techniques based on analysis in the time domain. The frequency error is calculated for a common channel conveying information which is known to the mobile terminal. Correction is then applied symbol by symbol to the input signal. This step entails using a complex product of frequency error and input signal, calculated using sine and cosine functions.
The principal drawback of the frequency error correction techniques explained above is the very complex calculations that they require when implemented on a CDMA mobile terminal. What is more, none of the above solutions is also intended to address the problem of channel estimate filtering.
Accordingly, to regulate the complexity of the calculations, the invention proposes to alleviate the drawbacks of the prior art previously cited by providing a frequency error correction method intended to be used in CDMA mobile radio terminals which is adaptive, depending on the value of the frequency error. The invention therefore provides three correction modes for frequency error correction. A closed-loop mode corresponds to very accurate frequency error correction, an open-loop mode corresponds to coarse correction, and a final mode is entirely devoid of frequency error correction. This system matches the complexity of the frequency error correction calculations to what is required, without compromising performance in terms of demodulation.